In conventional installations of lighting devices in combination with a motion sensor, as are known from stairwells in the prior art, for example, on activation of the motion sensor that phase of an AC voltage supply which is coupled to the motion sensor is coupled to the electronic control gear of the lighting device, said electronic control gear being connected downstream of the motion sensor. As a result, the lighting device is switched on.
Recently, electronic control gear have been disclosed, for example the so-called dual-power ECG by the applicant, in which a specific operating mode can be selected by means of a control input. In the case of the electronic control gear mentioned by way of example, it is possible in this context to operate only at least one LED or only at least one discharge lamp or both at the same time. In the case of such electronic control gear, the switching output of the motion sensor is coupled to the control input of the electronic control gear, while the electronic control gear itself is coupled to a phase of the AC voltage supply. If there is no defined signal present at the control input, an undesired signal can result at the control input owing to capacitive coupling of other live lines, and this undesired signal is interpreted incorrectly by the evaluation apparatus. There is therefore the risk of capacitive couplings in particular since the control line which is coupled to the control input and at whose other end the motion sensor is located is often laid parallel to the AC supply line of the electronic control gear.
A conventional evaluation apparatus which may be used in this context is illustrated schematically in FIG. 1. If a switch S1 is closed, a voltage drop occurs across a shunt resistor R1, with this voltage being proportional to the voltage at the control input SW. A voltage divider is formed by the ohmic resistors R1 and R2, between which a diode D1 is coupled for the purpose of rectification. Two varistors Var1 and Var2 serve the purpose of protecting the switch S1 from overvoltages. The part including the switch S1 and the ohmic resistor R1 is connected in parallel with the capacitor C1. The microcontroller μC retrieves the voltage drop across the shunt resistor R1 as a ratio of 1:10. This is because, in the evaluation apparatus illustrated in FIG. 1, the input impedance is only 10 kohms and losses of the evaluation apparatus can be minimized by the driving with a ratio of 1:10. In addition to the costs incurred for the varistors Var1, Var2, the design of the switch S1 is in this case problematic. Said switch S1 needs to be designed for surge pulses of 800 V.
An increase in the input impedance in order to reduce the losses is not an option since, as a result, the problems owing to capacitive charges on the control line would be increased. That is to say that if the input impedance is increased, the discharge of the line capacitance would be slower and would therefore have a disadvantageous effect on the evaluation of the control signal SW at the input of the microcontroller μC. An increase in the duty factor for the purpose of counteracting an increase in the input impedance would in turn result in the lengths of time for which current flows from the control input SW to the reference potential being increased, as a result of which the losses of the evaluation apparatus would again be increased. Moreover, it is disadvantageous that the evaluation apparatus uses two pins of the microcontroller μC. There are applications in which only a single pin can be provided for the evaluation apparatus.